Red blood cells carry oxygen from lungs to the whole body. The percent saturation of hemoglobin with oxygen depends on the partial pressure of oxygen. Higher partial pressure of oxygen means more hemoglobin can carry more oxygen, while lower partial pressure of oxygen means hemoglobin releases oxygen. The relationship is shown in the graph. Partial pressure of oxygen

In lungs = $\ce{11 kPa}$,
in muscles = $\ce{2 kPa}$

100% oxygen saturated hemoglobin carries: $\ce{20 mL oxygen/100 mL blood}$. While going to muscles from lungs, what is the amount of oxygen (in mL) released by hemoglobin per $\pu{100 mL}$ blood? Write it as an integer. graph

My progress

So, it seemed like too easy question to me.

As the graph says, Saturation of Hemoglobin with Oxygen is 100% when partial pressure of oxygen is $\pu{14 kPa}$ So, at $\pu{14 kPa}$ it carries $\ce{20 mL oxygen/100 mL blood}$

With this information, it can be said that at $\pu{11 kPa}$ and $\pu{2 kPa}$ it will carry $\pu{15.71 ml}$ and $\pu{2.85 ml}$ respectively.

Which are actually the amount of oxygen carried in lungs and muscles. So, while going to muscles from lungs, it should release

$\ce{15.71 - 2.85 = 12.86 ≈ 13 ml}$

So, $\pu{13 ml/ 100ml of blood}$

But I have confusion if this is correct reading this part

Higher partial pressure of oxygen means more hemoglobin can carry more oxygen, while lower partial pressure of oxygen means hemoglobin releases oxygen.

I am confused, if the way I found the difference is proper

So, I am not sure if that was the correct way. Is it the correct procedure ?

  • $\begingroup$ If you can solve this problem, why are you posting here? Don't you have an instructor to mark your work? This is a biology question and answer site, not a homework checking site. $\endgroup$
    – David
    Commented Jul 27, 2021 at 12:01
  • $\begingroup$ I’m voting to close this question because the question appears to be an attempt to use the site as a homework checking service. $\endgroup$
    – David
    Commented Jul 27, 2021 at 12:01
  • 1
    $\begingroup$ I don't have any tutor to check and I am practicing a olympiad questions which answers are never given. People here ask for efforts to add so I added how I progressed. But when I have done this, people are closing it. How funny $\endgroup$
    – Abrar
    Commented Jul 27, 2021 at 14:02
  • $\begingroup$ Your calculations are wrong and you don't seem to be using the values from the graph. For example, the graph does not show the hemoglobin 100% saturated at 14 kPa. $\endgroup$
    – Armand
    Commented Jul 27, 2021 at 14:21
  • 2
    $\begingroup$ I think this question demonstrates the sort of effort required under our homework policy; they've shown their work towards an answer. I am also, however, a bit concerned with this site being used as an "answer checker" service when answers are not provided, even if work is shown. Questions should be asked in ways that are useful to others. $\endgroup$
    – Bryan Krause
    Commented Jul 27, 2021 at 15:22

1 Answer 1


I'd solve differently, in a way that removes some imprecision in your estimates and incorrect assumptions.

We care about two points on the graph: saturation at 11 and 2 kPa. At 11 it reads approximately 96% saturated; at 2 it reads about 22% saturated. So we can say the difference from 11 kPa to 2 kPa is 74% of the total capacity of hemoglobin.

If we know the total capacity is 20 mL/100 mL; then 74% of 20 mL is 14.8 mL. As an integer approximation I'd write 15 mL.

You shouldn't be caring or using the 14 kPa number at all, it's completely irrelevant. As Armand points out, hemoglobin is not 100% saturated at 14 kPa. More importantly, the graph shows that the relationship between pressure and oxygen saturation is not linear. It seems like you've done your math by assuming 14 kPa is maximal and then there is a linear relationship so that at 7 kPa it is 7/14=half, at 2 kPa it's 2/14, etc. You could plot these assumptions out on the graph and you'll find it doesn't look like what is shown at all. It turns out to be not a completely terrible approximation, since your answer of 13 mL is fairly close to 15 mL, but it's not precise enough to get to the nearest integer.

  • $\begingroup$ Thanks for pointing out my problems. I never noticed those errors in my assumption. Thanks a lot 😇 $\endgroup$
    – Abrar
    Commented Jul 27, 2021 at 16:24

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